1. Field of the Invention
The present invention generally relates to a circuit for controlling scanning of light beams, an optical unit, and an image forming device. The present invention particularly relates to a circuit for controlling scanning of light beams so as to achieve desired scanning of a plurality of light beams, and further relates to an optical unit and an image forming device based on such a control circuit.
2. Description of the Related Art
Laser beam printers are required to be faster and to be better in terms of image resolution. To meet such requirements, scanning of a laser beam needs to be faster, which is achieved by rotating a polygon mirror of an optical unit at a faster rate. A rotation rate of a polygon mirror is around 30,000 rpm in a currently available optical unit, and this rate is close to an upper limit when physical and mechanical constraints are taken into account. Against this background, a multi-beam optical unit has been developed in order to scan a plurality of laser beams.
The multi-beam optical unit scans a plurality of laser beams on a photosensitive drum while slightly displacing each laser beam from the others, thereby create a latent image on the photosensitive drum. If relative positional relations between the laser beams are not correct, it results in a poor image quality. Relation of scan positions between laser beams thus needs to be controlled with accuracy.
FIG. 1 is a block diagram of a related-art laser printer.
The laser printer 1 includes a printer-sheet conveyor mechanism 2, a photosensitive drum 3, a charging device 4, an optical unit 5, a developing unit 6, a transfer unit 7, a fixing unit 8, and a video controller 9.
A printer sheet 10 is carried by the printer-sheet conveyor mechanism 2 in a direction shown by an arrow A. The printer sheet 10 first comes in contact with the photosensitive drum 3, which rotates in a direction shown by an arrow B. The photosensitive drum 3 is electrically charged by the charging device 4, and, rotates in the direction B. The optical unit 5 directs laser beams L1 and L2 onto the photosensitive drum 3 so as to create a latent image on a surface thereof.
The photosensitive drum 3 having the latent image created thereon is further rotated in the direction B, and has a toner image developed thereon by the developing unit 6. The transfer unit 7 then transfers the toner image from the photosensitive drum 3 to the printer sheet 10.
The printer-sheet conveyor mechanism 2 further carries the printer sheet 10 in the direction A, so that the printer sheet 10 is supplied to the fixing unit 8. The fixing unit 8 fixes the transferred toner image permanently on the printer sheet 10 by applying heat, for example. The printer sheet 10 having the toner image fixed thereon is further carried in the direction A by the printer-sheet conveyor mechanism 2 until it is ejected.
In the following, a description of the optical unit 5 will be given.
The optical unit 5 includes laser diodes D1 and D2, a polygon mirror 11, a motor 12, a motor driver 13, mirrors 14 and 15, a laser-beam-reference-position detecting unit 16, laser-power-adjustment beam-detection units 17 and 18, an optical control unit 19, a mechanism control unit 20, a power unit 21, and switches SW1 through SW4.
The laser diodes D1 and D2 are connected to the optical control unit 19, and generate laser beams L1 and L2, respectively. The laser beams L1 and L2 hit the polygon mirror 11.
The polygon mirror 11 is rotated by the motor 12 in a direction shown by an arrow C at a constant rotation rate. The motor driver 13 controls the motor 12 to rotate at a constant rate such as 3,000 rpm. The laser beams L1 and L2 are reflected by the rotating polygon mirror 11 so that they are scanned in the direction C.
The polygon mirror 11 directs the laser beams L1 and L2 to the mirror 15 first. The mirror 15 reflects the laser beams L1 and L2 coming from the polygon mirror 11 so as to direct them to the laser-beam-reference-position detecting unit 16. After the laser beams L1 and L2 illuminate the laser-beam-reference-position detecting unit 16, the polygon mirror 11 further rotates in the direction C (i.e., the direction shown by the arrow C). The laser beams L1 and L2 are scanned in a direction indicated by an arrow D, and hit the mirror 14. The mirror 14 reflects the laser beams L1 and L2, and directs them to the photosensitive drum 3.
As the polygon mirror 11 rotates in the direction C, the laser beams L1 and L2 are scanned in the direction D, so that they move on the mirror 14 in a direction indicated by an arrow E. As the laser beams L1 and L2 sweep in the direction E on the mirror 14, their reflections are scanned in a direction shown by an arrow F on the photosensitive drum 3. As the scanning of the laser beams L1 and L2 on the photosensitive drum 3 progresses, the laser beams L1 and L2 are controlled to be turned on or off appropriately.
The laser-beam-reference-position detecting unit 16, which detects the laser beams L1 and L2 as previously described, supplies a detection signal to the optical control unit 19.
The optical control unit 19 detects positions of the laser beams L1 and L2 on the photosensitive drum 3 based on the detection signal supplied from the laser-beam-reference-position detecting unit 16.
The laser beams L1 and L2 are also supplied to the laser-power-adjustment beam-detection units 17 and 18, respectively. The laser-power-adjustment beam-detection units 17 and 18 supply detection signals indicative of intensities of the laser beams L1 and L2, respectively, to the optical control unit 19. The optical control unit 19 monitors the intensities of the laser beams L1 and L2, and attends to power control so as to achieve constant beam intensity.
The optical control unit 19 includes laser control circuits 22 and 23 and an optical control circuit 24. The laser control circuits 22 and 23 control the laser diodes D1 and D2, respectively, based on the detection signals supplied from the laser-power-adjustment beam-detection units 17 and 18, respectively.
The optical control circuit 24 receives a video signal from the video controller 9 where the video signal represents an image to be reproduced on a sheet. Further, the optical control circuit 24 receives the detection signal from the laser-beam-reference-position detecting unit 16. In response to the detection signal from the laser-beam-reference-position detecting unit 16, the optical control circuit 24 controls an on/off state of the laser beams L1 and L2 in accordance with the video signal supplied from the video controller 9.
The mechanism control unit 20 attends to control of various mechanisms such as drive control of the printer-sheet conveyor mechanism 2 and rotation control of the photosensitive drum 3. Further, the mechanism control unit 20 includes relays R1 through 5, which are switched according to the switches SW1 through SW4 that detect an open/close status of a stack cover, a front cover, and an eject cover as well as a presence/absence of a transport unit. In the mechanism control unit 20, the relays R1 through R5 are turned on if the stack cover, the front cover, and the eject cover are all closed, and if the transport unit is present. As the relays R1 through R5 are turned on, power is supplied from the power unit 21 to the optical control unit 19.
In the following, the optical control circuit 24 will be described in connection with timing control of the laser beams L1 and L2.
FIG. 2 is an illustrative drawing for explaining a displacement between the laser beams L1 and L2 in their scan direction shown by an arrow F. FIG. 3 is an illustrative drawing for explaining a displacement between the laser beams L1 and L2 in their sub-scan direction shown by an arrow B. FIGS. 4A and 4B are illustrative drawings showing a positional relation between the laser beams L1 and L2.
As shown in FIG. 2, the laser beam L1 is scanned ahead of the laser beam L2 in a main scan direction shown by the arrow F. As shown in FIG. 3, also, the laser beams L1 and L2 are displaced from each other in the sub-scan direction B by one scan line, and are positioned separately on adjacent scan lines.
Namely, the laser beams L1 and L2 have a gap L therebetween in the main scan direction F, and are separated from each other by one line in the sub-scan direction B as shown in FIG. 4A. With this relative positioning being maintained, the laser beams L1 and L2 are scanned on the photosensitive drum 3.
It should be noted that when dots are to be generated in a line along the direction B as shown in FIG. 4B, a time T from a synchronization point BD to a position P0 needs to be accurately controlled.
In the following, a circuit for controlling timings of the laser beams L1 and L2 will be described.
FIG. 5 is a block diagram of the related-art optical control circuit.
The optical control circuit 24 includes a video-data splitting circuit 25, parallel-to-serial-conversion circuits 26 and 27, a detection-signal processing circuit 28, an oscillator circuit 29, synchronization circuits 30 and 31, delay units 32 and 33, and 1/N frequency dividing units 34 and 35.
The video-data splitting circuit 25 receives the video data from the video controller 9, and distributes video lines alternately to the parallel-to-serial-conversion circuit 26 or to the parallel-to-serial-conversion circuit 27.
The parallel-to-serial-conversion circuit 26 attends to parallel-to-serial conversion of the video data supplied from the video-data splitting circuit 25 in synchronism with a clock signal supplied from the 1/N frequency dividing unit 34. Serial data output from the parallel-to-serial-conversion circuit 26 is supplied to the laser control circuit 22. Based on the supplied serial data, the laser control circuit 22 controls the laser diode D1.
The parallel-to-serial-conversion circuit 27 attends to parallel-to-serial conversion of the video data supplied from the video-data splitting circuit 25 in synchronism with a clock signal supplied from the 1/N frequency dividing unit 35. Serial data output from the parallel-to-serial-conversion circuit 27 is supplied to the laser control circuit 23. Based on the supplied serial data, the laser control circuit 23 controls the laser diode D2.
The laser beams L1 and L2 output from the laser diodes D1 and D2 hit the polygon mirror 11. The polygon mirror 11 reflects the laser beams L1 and L2, and directs them to the laser-beam-reference-position detecting unit 16. Thereafter, the laser beams L1 and L2 are scanned over the photosensitive drum 3.
The detection signal from the laser-beam-reference-position detecting unit 16 is supplied to the detection-signal processing circuit 28. Based on the supplied detection signal, the detection-signal processing circuit 28 controls the video-data distribution timing of the video-data splitting circuit 25. 28 supplies detection pulses to the synchronization circuits 30 and 31 in response to the detection signal from the laser-beam-reference-position detecting unit 16.
Each of the synchronization circuits 30 and 31 receives a clock signal from the oscillator circuit 29 where the clock signal has a frequency N times as high as that of a video clock. In response to the detection pulse from the detection-signal processing circuit 28, each of the synchronization circuits 30 and 31 outputs the clock signal supplied from the oscillator circuit 29.
The clock signals output from the synchronization circuits 30 and 31 are supplied to the delay units 32 and 33, respectively. The delay units 32 and 33 delay the supplied clock signals by a predetermined delay time, and supply the delayed clock signals to the 1/N frequency dividing units 34 and 35, respectively.
The delay time introduced by the delay units 32 and 33 should be equal to a time difference between when the laser-beam-reference-position detecting unit 16 detects a laser beam and when the laser beam reaches a position where the video data should be printed. Namely, the delay time should be equivalent to the time T shown in FIG. 4B. The 1/N frequency dividing units 34 and 35 divide by N the frequency of the clock signal supplied from the delay units 32 and 33, respectively, thereby generating clock signals having the same cycle as the video-data clock. The generated clock signals are supplied to the parallel-to-serial-conversion circuits 26 and 27.
In response to the respective clock signals supplied from the 1/N frequency dividing units 34 and 35, the parallel-to-serial-conversion circuits 26 and 27 convert parallel video data to serial data when the parallel video data is supplied form the video-data splittin to the laser control circuits 22 and 23.
In the related-art configuration described above, a timing of a laser-beam scan is marked by a detection signal that is generated by detecting a laser beam at a reference position. In response to this detection signal, a clock signal with a frequency thereof N times as high as the frequency of video data is extracted, and its frequency is divided by N to generate a video clock. In this configuration, a timing of the video data is adjustable only by a minimum shift equal to one clock cycle of the clock signal that has N times the frequency of the video clock.
If the frequency of the clock signal is increased with an aim of achieving finer adjustment of clock timings, electrical-circuit components need to be changed to those adapted for use in high frequencies, and a measure has to be taken against electromagnetic fields. Such needs result in more expensive devices.
Accordingly, there is a need for a circuit for controlling scanning of light beams which can achieve finer timing adjustment of scanning of light beams, and, also, there is a need for an optical unit and an image forming device employing such a control circuit.
Accordingly, it is a general object of the present invention to provide a scheme for controlling scanning of light beams which can satisfy the need described above.
It is another and more specific object of the present invention to provide a circuit for controlling scanning of light beams which can achieve finer timing adjustment of scanning of light beams.
In order to achieve the above objects according to the present invention, a circuit for controlling scanning of light includes a detection-signal processing unit which generates a detection signal indicative of a timing at which a light beam hits a predetermined reference position, and a clock-generation unit which generates a clock signal in synchronism with the detection signal such that a timing of the clock signal is adjustable by the detection signal independently of a length of one clock cycle of the clock signal, wherein the clock signal synchronizes the light beam.
In the control circuit described above, the clock-generation unit such as a clock generator generates the clock signal in synchronism with the detection signal. The timing of the clock signal is thus free from an undesirable displacement that is found in the related-art configuration in which the adjustment of the clock signal is dependent upon the length of the clock cycle of the clock signal.
It is yet another object of the present invention to provide an optical unit and an image forming device employing such a control circuit.
The object described above is achieved by an optical unit or an image forming device including a light-beam generating unit which generates a light beam, a light-beam scanning unit which scans the light beam, a detection-signal processing unit which generates a detection signal indicative of a timing at which the light beam hits a predetermined reference position, and a clock-generation unit which generates a clock signal in synchronism with the detection signal such that a timing of the clock signal is adjustable by the detection signal independently of a length of one clock cycle of the clock signal, wherein the clock signal synchronizes the light beam.
Other objects and further features of the present invention will be apparent from the following detailed description when read in conjunction with the accompanying drawings.